There has been known a substrate processing apparatus in which a semiconductor wafer (hereinafter, simply referred to as “wafer”) as a substrate is accommodated in a chamber and desired processes such as a film-forming process or a plasma process is performed with respect to the wafer using a process gas introduced into the chamber or plasma generated inside the chamber. In such a substrate processing apparatus, in order to perform the desired processed on the wafer, the wafer is mounted on a susceptor as a stage disposed inside the chamber.
It is necessary to mount the wafer at a predetermined position on a wafer-mounting surface (hereinafter, simply referred to as “mounting surface”) of the susceptor. There may be a case where the wafer thus mounted is misaligned from the predetermined position on the mounting surface. In this case, for example, when a thermal CVD (chemical vapor deposition) process or ALD (atomic layer deposition) process as the film-forming process is performed on the wafer, the wafer is misaligned from a heater incorporated in the susceptor so that the heater fails to uniformly heat the wafer, thereby causing the thickness of a film formed on the wafer to be non-uniform. Furthermore, for example, when an etching process as the plasma process is performed on the wafer, an impedance deviation caused by the misalignment of the wafer occurs at an edge portion of the wafer. This makes the thickness of a sheath formed on a surface of the wafer non-uniform, which makes the etched amount in each portion of the wafer non-uniform.
To address this, there has been proposed a technique in which a pocket composed of a recess having a diameter slightly larger than that of the wafer is installed in the mounting surface, and projections for positioning the wafer are formed at a side surface of the pocket. When the wafer is received in the pocket, the wafer descends along a tapered surface formed in each of the projections so that the wafer is received at a proper position inside the pocket.
In general, however, when a wafer is mounted on a susceptor, the wafer is first delivered from a transfer arm to a plurality of lift pins formed to protrude upwardly from a mounting surface of the susceptor. The transfer arm is withdrawn from a chamber, and subsequently, the lift pins descend or the susceptor ascends to mount the wafer on the susceptor.
However, when the wafer is mounted on the susceptor, there may be a case where the wafer does not come into an uniform contact with the mounting surface and only a portion thereof is in contact with the mounting surface. In this case, there is a problem that even if the pocket is formed in the mounting surface, a drag force generated from the mounting surface is exerted on the wafer at a slight angle with respect to a vertical direction of the wafer and a component of the drag force in a direction parallel to the wafer is exerted on the wafer as a moving force so that the wafer may be misaligned from the predetermined position on the mounting surface of the susceptor. In particular, if the wafer has a larger diameter, an increase in a contact area between the wafer and the mounting surface also involves an increase in the drag force and ultimately the component thereof, so that there may be concern that the misalignment of the wafer becomes remarkable.